An ignition plug used for a combustion apparatus such as an internal combustion engine includes, for example, a center electrode extending in the axial direction, an insulator provided around the center electrode, a tubular metallic shell provided around the insulator, and a ground electrode having a base end portion joined to a forward end portion of the metallic shell. Through application of high voltage to the center electrode, spark discharge is produced at a gap formed between the center electrode and the ground electrode, whereby a fuel-air mixture is ignited.
In recent years, a technique for improving ignition performance has been proposed (see, for example, Patent Document 1, etc.). In the proposed technique, high-frequency electric power is supplied to the gap in place of high voltage so as to produce high-frequency plasma to thereby ignite a fuel-air mixture. Also, there has been proposed a technique of producing high-frequency plasma by supplying high-frequency electric power to spark generated through application of high voltage.
In addition, in order to enhance durability and/or ignition performance, a tip formed of a noble metal alloy or the like may be joined to a forward end portion of the center electrode. In general, such a tip is joined to the center electrode via a fusion portion which is formed by laser welding from the metal which forms the center electrode and the metal which forms the tip (see, for example, Patent Document 2, etc.).